iOS/Android/BlackBerry: People fall asleep on public transit, whether intentionally or otherwise. Mobile app TravAlert keeps track of your location without killing your battery, and then wakes you up in time to get off at the right stop.

TravAlert uses a timed GPS ping schedule to watch your location while you're traveling on major cities' trains, subways, and buses, checking in every so often to make sure you're still on schedule. If you head into a tunnel or another area without service, TravAlert estimates where you'll be based on your last GPS position. When you're a distance you define from your stop, TravAlert alerts you in the way you choose to wake up and get your gear together. It could also be handy when you're on a noisy line and can't quite hear the conductor.

TravAlert is a $1.99 purchase for iOS and Android, and also available for BlackBerry.